1. Field of the Invention
The present invention relates to a tungsten based heavy alloy, and more particularly to a tungsten based heavy alloy having a W--Ni--Fe based composition containing traces of lanthanum or calcium, thereby capable of exhibiting the high toughness, irrespective of the content of impurities such as phosphorous and sulfur contained therein, the cooling rate after the sintering treatment and the re-heating treatment, and a method for manufacturing thereof.
2. Description of the Prior Art
Generally, such a tungsten based alloy consists of about 90% tungsten and the balance nickel and iron(copper). Since tungsten is a metal having a high melting point, the tungsten based alloy is produced by using a liquid sintering process which is one of powder metallurgies.
Tungsten based heavy alloys are widely used in various technical fields, to produce weights of gyroscopes, balanced supports of aircraft, containers for containing radioactive materials, vibration attenuators, warheads for breaking armor plates, and etc.
In terms of the mechanical properties, such tungsten based heavy alloys have relatively superior strength and elongation, to other alloys. However, toughness of tungsten based heavy alloys may vary greatly, depending upon the used manufacturing process and the used powder. In particular, heavy alloys made of a powder in which impurities such as phosphorous and sulfur were insufficiently removed during the manufacturing processes exhibit deteriorated mechanical properties involving toughness. As a result, such heavy alloys can not be used in manufacturing products requiring high mechanical properties, such as warheads for breaking armor plates.
It has been known that the deterioration of the mechanical property of heavy alloys due to the impurities was caused by the boundary segregation of the impurities. Accordingly, there have been researches for avoiding the boundary segregation of impurities presented in heavy alloys, in order to prevent the mechanical properties of heavy alloys from being deteriorated.
For example, a method for avoiding the boundary segregation of impurities has been known, which utilized the fact that the boundary segregation of impurities is reduced as the heat treatment temperature of heavy alloy increases. The method can be mainly applied to a heavy alloy containing impurities at a small amount of several hundred ppm and comprises water-cooling the heavy alloy at a high temperature of about 1,000.degree. C., so as to avoid the boundary segregation and thus the deterioration of toughness.
It is also known that as the alloy is subjected to an aging at a temperature of about 500.degree. C. to 600.degree. C., as a subsequent heat treatment, the mechanical property of the treated alloy is improved. However, this aging treatment causes the displacement of impurities dispersed in the alloy structure to crystalline boundaries of the structure, thereby resulting in the boundary segregation of the impurities. Consequently, the problem of the boundary segregation of the impurities occurs again. Thus, it is difficult to improve the mechanical property of heavy alloys, by using the water cooling at a high temperature and the aging treatment at a low temperature.
For producing a heavy alloy having high toughness, therefore, it is necessary to provide a heavy alloy of a new type which has a composition capable of improving the mechanical property irrespective of the substantial content of impurities and the heat treatment.